DFT accounts for electron correlations indirectly. The Hohenberg-Kohn theorem proves that ground state properties can be obtained exactly from three-dimensional spatial density rather than multidimensional wavefunctions and DFT replaces coulomb interaction with a density functional, Figure 2.1. Kohn-Sham (KS) DFT ensures that density corresponds to a wavefunction. KS DFT is an effective single-particle theory where interacting electrons in a static external field are treated as non-interacting electrons moving in an effective potential. HF can be viewed as a special form of DFT that includes exact Pauli exchange with no electron correlation. As illustrated below, HF and DFT give different single-particle pictures. DFT is computationally inexpensive and works particularly well with extended systems that do not undergo complex chemical changes.